An optical encoder is used as a rotation angle detecting device, but is used in a restricted temperature environment, complex in structure, and expensive. Therefore, as a rotation angle detecting device which is simple in structure, inexpensive, and durable even in a high temperature environment, there have conventionally been devised rotation angle detecting devices using a change in permeance of a gap between a rotor and a stator. For example, there are disclosed a rotation angle detecting device including exciting windings of two phases and an output winding of one phase, a rotation angle detecting device including an exciting winding of one phase and output windings of two phases, and a rotation angle detecting device having the number of turns varying so as to match with a sine wave (see, for example, PTLs 1 to 4).
For example, the rotation angle detecting device disclosed in PTL 1 includes a stator formed of an iron core with teeth, around which an exciting winding of one or more phases and output windings of two or more phases are wound, and a rotor with a salient pole. As a shaft angle multiplier increases, the number of teeth also increases in proportion.